U.S. patent number 8,576,451 [Application Number 13/178,962] was granted by the patent office on 2013-11-05 for versatile moire-free halftone geometry that uses frequency vector shearing.
This patent grant is currently assigned to Xerox Corporation. The grantee listed for this patent is Zhigang Fan, Robert Paul Loce, Shen-Ge Wang. Invention is credited to Zhigang Fan, Robert Paul Loce, Shen-Ge Wang.
United States Patent |
8,576,451 |
Wang , et al. |
November 5, 2013 |
Versatile moire-free halftone geometry that uses frequency vector
shearing
Abstract
As set forth herein, computer-implemented methods and systems
facilitate the generation of halftone screens for moire-free color
halftoning. A first fundamental frequency vector and a second
fundamental frequency vector of a halftone cell are sheared using a
selected shearing value. The shearing value is selected as an
offset in a fast scanning or slow scanning direction. The selected
shearing value satisfies various moire-free conditions associated
with the identified frequency vectors and is capable of being
selected for multiple halftone screens. The halftone screens
generated using the sheared frequency vectors are used for
moire-free halftoning.
Inventors: |
Wang; Shen-Ge (Fairport,
NY), Fan; Zhigang (Webster, NY), Loce; Robert Paul
(Webster, NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
Wang; Shen-Ge
Fan; Zhigang
Loce; Robert Paul |
Fairport
Webster
Webster |
NY
NY
NY |
US
US
US |
|
|
Assignee: |
Xerox Corporation (Norwalk,
CT)
|
Family
ID: |
47438509 |
Appl.
No.: |
13/178,962 |
Filed: |
July 8, 2011 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20130010336 A1 |
Jan 10, 2013 |
|
Current U.S.
Class: |
358/3.06;
358/1.9; 358/1.15; 358/3.03; 345/596; 358/3.2; 399/180;
347/131 |
Current CPC
Class: |
H04N
1/52 (20130101); H04N 1/405 (20130101) |
Current International
Class: |
H04N
1/405 (20060101); G03G 15/01 (20060101); G03G
13/04 (20060101); G09G 5/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Wang, et al. "Non-Orthogonal Halftone Screens," Proc. NIP18:
International Conference on Digital Printing Technologies, pp.
578-584, 2002. cited by applicant .
Ulichney, "Digital Halftoning," The MIT Press, pp. 117-126, 1988.
cited by applicant .
Turbek, et al. "Comparison of Hexagonal and Square Dot Centers for
EP Halftones," PICS 2000, pp. 321-325. cited by applicant.
|
Primary Examiner: Kau; Steven
Attorney, Agent or Firm: Fay Sharpe LLP
Claims
What is claimed is:
1. A computer-implemented method for generating halftone screens
for moire-free color halftoning, comprising: identifying a first
color halftone screen having halftone cells defined by a first
fundamental frequency vector (V.sub.1) and a second fundamental
frequency vector (V.sub.2); selecting at least one shearing value
(s) representative of an offset in at least one of a fast scan
direction and a slow scan direction; shearing at least one of the
first fundamental frequency vector (V.sub.1) and the second
fundamental frequency vector (V.sub.2) for the first halftone
screen by application of the shearing value (s) associated
therewith; and generating a first sheared color halftone screen in
accordance with at least one of the sheared first fundamental
frequency vector (V.sub.1) and the second fundamental frequency
vector (V.sub.2) so as to avoid moire image effects.
2. The method of claim 1, wherein the sheared first fundamental
frequency vector V.sub.1(f.sub.x1,f.sub.y1) and the sheared second
fundamental frequency vector V.sub.2 (f.sub.x2,f.sub.y2) are
expressed as:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times. ##EQU00008##
.times..times..times..times..times..times..times..times.
##EQU00008.2## where (.epsilon.) is the selected shearing value
(s), A.sub.o is an area of a halftone cell, and (x.sub.o, y.sub.o)
is a nominal coordinate corresponding to the output location of the
at least one sheared vector.
3. The method of claim 1, wherein the at least one shearing value
(s) is a line-to-line offset in the start-of-scan signal for a
raster line, further comprising linearly increasing the shearing
value (s) responsive to a change in the raster line.
4. The method of claim 1, wherein the at least one shearing value
(s) is an angular offset of at least one of the first fundamental
frequency vector (V.sub.1) and the second fundamental frequency
vector (V.sub.2).
5. The method of claim 1, wherein the at least one shearing value
(s) is a periodic offset in the start-of-scan signal for a raster
line, and wherein the periodic offset is generated responsive to
alternating between two or more phases of an associated pixel
clock.
6. The method of claim 1, wherein selecting a shearing value (s)
further comprises determining a shearing offset value
(.epsilon..sub.x) in an x-direction and a shearing offset value
(.epsilon..sub.y) in a y-direction, and wherein the shearing
comprises applying the offset (.epsilon..sub.x) in the x-direction
and applying the offset (.epsilon..sub.y) in the y-direction to at
least one of V.sub.1 and V.sub.2.
7. The method of claim 2, wherein the first fundamental frequency
vector (V.sub.1) is defined as (V.sub.c1) and the second
fundamental frequency vector (V.sub.2) is defined as (V.sub.c2),
further comprising: identifying a second color halftone screen
having halftone cells defined by a first fundamental frequency
vector (V.sub.m1) and a second fundamental frequency vector
(V.sub.m2); identifying a third color halftone screen having
halftone cells defined by a first fundamental frequency vector
(V.sub.k1) and a second fundamental frequency vector (V.sub.k2);
selecting a shearing value (.epsilon..sub.m) representative of an
offset in at least one of a fast scan direction and a slow scan
direction corresponding to the second color halftone screen, and a
shearing value (.epsilon..sub.k) corresponding to the third color
halftone screen; shearing at least one of the first fundamental
frequency vector (V.sub.m1) and the second fundamental frequency
vector (V.sub.m2) for the second halftone screen by application of
the shearing value (.epsilon..sub.m) associated therewith, so as to
generate a second sheared color halftone screen for avoiding moire
image effects; shearing at least one of the first fundamental
frequency vector (V.sub.k1) and the second fundamental frequency
vector (V.sub.k2) for the third halftone screen by application of
the shearing value (.epsilon..sub.k) associated therewith, so as to
generate a third sheared color halftone screen for avoiding moire
image effects; and outputting a set of sheared color halftone
screens comprising the first, second, and third sheared color
halftone screens.
8. The method of claim 7, wherein selecting the shearing values
(.epsilon..sub.c, .epsilon..sub.m, .epsilon..sub.k) comprises
satisfying the following:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times. ##EQU00009##
.times..times..times..times..times..times..times. ##EQU00009.2##
where A represents an area of a halftone cell associated with the
identified halftone screen, and (x.sub.o, y.sub.o) is a nominal
coordinate corresponding to the output location of the at least one
sheared vector.
9. A computer-implemented method for generating a set of sheared
color halftone screens for moire-free color halftoning, comprising:
identifying a first color halftone screen having halftone cells
defined by a first fundamental frequency vector (V.sub.c1) and a
second fundamental frequency vector (V.sub.c2), a second color
halftone screen defined by a first fundamental frequency vector
(V.sub.m1) and a second fundamental frequency vector (V.sub.m2),
and a third color halftone screen defined by a first fundamental
vector (V.sub.k1) and a second fundamental frequency vector
(V.sub.k2); selecting a shearing value (s.sub.c, s.sub.m, s.sub.k)
corresponding to an offset in at least one of a fast scan direction
and a slow scan direction for each of the first, second, and third
color halftone screens; shearing at least one of the first
fundamental frequency vectors (V.sub.c1, V.sub.m1, V.sub.k1) and
the second fundamental frequency vectors (V.sub.c2, V.sub.m2,
V.sub.k2) for each of the first, second, and third halftone screens
by application of the shearing values (s.sub.c, s.sub.m, s.sub.k)
corresponding thereto; generating a set of sheared color halftone
screens in accordance with at least one of the sheared first
fundamental frequency vectors (V.sub.c1, V.sub.m1, V.sub.k1) and
the second fundamental frequency vectors (V.sub.c2, V.sub.m2,
V.sub.k2) so as to avoid moire image effects; and outputting the
set of sheared color halftone screens.
10. The method of claim 9, wherein the shearing values (s.sub.c,
s.sub.m, s.sub.k) are at least one of: a line-to-line offset in the
start-of-scan signal for a raster line, an angular offset of at
least one of the first fundamental frequency vectors (V.sub.c1,
V.sub.m1, V.sub.k1) and the second fundamental frequency vectors
(V.sub.c2, V.sub.m2, V.sub.k2), and a periodic offset in the
start-of-scan signal for a raster line that is generated responsive
to alternating between two or more phases of an associated pixel
clock.
11. The method of claim 9, wherein selecting the shearing values
(s.sub.c, s.sub.m, s.sub.k) further comprises determining a
respective shearing offset value (.epsilon..sub.cx,
.epsilon..sub.mx, .epsilon..sub.kx) in an x-direction and a
respective shearing offset value (.epsilon..sub.cy,
.epsilon..sub.my, .epsilon..sub.ky) in a y-direction, and wherein
the shearing comprises applying the shearing offset values
(.epsilon..sub.cy, .epsilon..sub.my, .epsilon..sub.ky) in the
x-direction and applying the shearing offset values
(.epsilon..sub.cy, .epsilon..sub.my, .epsilon..sub.ky) in the
y-direction to a corresponding at least one of the first
fundamental frequency vectors (V.sub.c1, V.sub.m1, V.sub.k1) and
the second fundamental frequency vectors (V.sub.c2, V.sub.m2,
V.sub.k2) for each of the first, second, and third halftone
screens.
12. The method of claim 9, wherein selecting a shearing value
(s.sub.c, s.sub.m, s.sub.k) comprises satisfying the following:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times. ##EQU00010##
.times..times..times..times..times..times. ##EQU00010.2## where
(.epsilon..sub.c, .epsilon..sub.m, .epsilon..sub.k) represent the
selected shearing values (s.sub.c, s.sub.m, s.sub.k), A represents
an area of a halftone cell associated with the identified halftone
screen, and (x.sub.o, y.sub.o) is a nominal coordinate
corresponding to the output location of the at least one sheared
vector.
13. The method of claim 11, wherein selecting an offset value
(.epsilon..sub.cx, .epsilon..sub.mx, .epsilon..sub.kx,
.epsilon..sub.cy, .epsilon..sub.my, .epsilon..sub.ky) comprises
satisfying the following:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times..times..times..times..-
times..times..times..times..times..times..times..times..times..times..time-
s..times..times..times..times..times..times..times..times.
##EQU00011##
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times. ##EQU00011.2## where A
represents an area of a halftone cell associated with the
identified halftone screen, and (x.sub.o, y.sub.o) is a nominal
coordinate corresponding to the output location of the at least one
sheared vector.
14. A system that facilitates generating halftone screens for
moire-free color halftoning, comprising: a processor operable to:
identify a first color halftone screen having halftone cells
defined by a first fundamental frequency vector (V.sub.1) and a
second fundamental frequency vector (V.sub.2), select at least one
shearing value (s) representative of an offset in at least one of a
fast scan direction and a slow scan direction, and shear at least
one of the first fundamental frequency vector (V.sub.1) and the
second fundamental frequency vector (V.sub.2) for the first
halftone screen by application of the shearing value (s) associated
therewith, generate a first sheared color halftone screen in
accordance with at least one of the sheared first fundamental
frequency vector (V.sub.1) and the second fundamental frequency
vector (V.sub.2) so as to avoid moire image effects; and a printer
that prints a halftone image using the first sheared color halftone
screen.
15. The system of claim 14, wherein the sheared first fundamental
frequency vector V.sub.1(f.sub.x1,f.sub.y1) and the sheared second
fundamental frequency vector V.sub.2 (f.sub.x2,f.sub.y2) are
expressed as:
.times..times..times..times..times..times..times..times..times..times-
..times..times..times..times..times..times..times..times..times.
##EQU00012##
.times..times..times..times..times..times..times..times.
##EQU00012.2## where (.epsilon.) is the selected shearing value
(s), A.sub.o is an area of a halftone cell, and (x.sub.o, y.sub.o)
is a nominal coordinate corresponding to the output location of the
at least one sheared vector.
16. The system of claim 14, wherein the at least one shearing value
(s) is at least one of: a line-to-line offset in the start-of-scan
signal for a raster line, and an angular offset of at least one of
the first fundamental frequency vector (V.sub.1) and the second
fundamental frequency vector (V.sub.2).
17. The system of claim 14, wherein the at least one shearing value
(s) is a periodic offset in the start-of-scan signal for a raster
line that is generated responsive to alternating between two or
more phases of an associated pixel clock.
18. The system of claim 14, wherein selecting a shearing value (s)
further comprises determining a shearing offset value
(.epsilon..sub.x) in an x-direction and a shearing offset value
(.epsilon..sub.y) in a y-direction, and wherein the shearing
comprises applying the offset (.epsilon..sub.x) in the x-direction
and applying the offset (.epsilon..sub.y) in the y-direction to at
least one of V.sub.1 and V.sub.2.
19. The system of claim 15, wherein the first fundamental frequency
vector (V.sub.1) is defined as (V.sub.c1) and the second
fundamental frequency vector (V.sub.2) is defined as (V.sub.c2),
the processor further operable to: identify a second color halftone
screen having non-orthogonal halftone cells defined by a first
fundamental frequency vector (V.sub.m1) and a second fundamental
frequency vector (V.sub.m2); identify a third color halftone screen
having non-orthogonal halftone cells defined by a first fundamental
frequency vector (V.sub.k1) and a second fundamental frequency
vector (V.sub.k2); select a shearing value (.epsilon..sub.m)
representative of an offset in at least one of a fast scan
direction and a slow scan direction corresponding to the second
color halftone screen, and a shearing value (.epsilon..sub.k)
corresponding to the third color halftone screen; shear at least
one of the first fundamental frequency vector (V.sub.m1) and the
second fundamental frequency vector (V.sub.m2) for the second
halftone screen by application of the shearing value
(.epsilon..sub.m) associated therewith, so as to generate a second
sheared color halftone screen for avoiding moire image effects;
shear at least one of the first fundamental frequency vector
(V.sub.k1) and the second fundamental frequency vector (V.sub.k2)
for the third halftone screen by application of the shearing value
(.epsilon..sub.k) associated therewith, so as to generate a third
sheared color halftone screen for avoiding moire image effects; and
output a set of sheared color halftone screens comprising the
first, second, and third sheared color halftone screens.
20. The system of claim 19, wherein selecting the shearing values
(s.sub.c, s.sub.m, s.sub.k) comprises satisfying the following:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times. ##EQU00013##
.times..times..times..times..times..times..times. ##EQU00013.2##
where (.epsilon..sub.c, .epsilon..sub.m, .epsilon..sub.k) represent
selected shearing values (s.sub.c, s.sub.m, s.sub.k), A represents
an area of a halftone cell associated with the identified halftone
screen, and (x.sub.o, y.sub.o) is a nominal coordinate
corresponding to the output location of the at least one sheared
vector.
Description
CROSS REFERENCE TO RELATED PATENTS AND APPLICATIONS
U.S. Patent Application No. 2008/0130056, filed Jun. 5, 2008,
entitled "Rosette Printing With Up To 5 Colors" by Wang the
disclosure of which is incorporated herein by reference in its
entirety.
U.S. Patent Application No. 2008/0130054, filed Jun. 5, 2008,
entitled "N-Color Printing With Hexagonal Rosettes" to Wang, the
disclosure of which is incorporated herein by reference in its
entirety.
TECHNICAL FIELD
The presently disclosed embodiments are directed toward methods and
systems for printing, reproducing or displaying images. More
particularly, the teachings disclosed herein are applicable to
methods and apparatuses wherein moire-free halftone geometries are
implemented.
BACKGROUND
With the advent of inexpensive digital color printers, methods and
systems of color digital halftoning have become increasingly
important in the reproduction of printed or displayed images
possessing continuous color tones. It is well understood that most
digital color printers operate in a binary mode, i.e. for each
color separation, a corresponding color spot is either printed or
not printed at a specified location or pixel. Digital halftoning
controls the printing of color spots, where the spatial averaging
of the printed color spots by either a human visual system or a
viewing instrument, provide the illusion of the required continuous
color tones.
The most common halftone technique is screening, which compares the
required continuous color tone level of each pixel for each color
separation with one or more predetermined threshold levels. The
predetermined threshold levels are typically defined for a
rectangular cell that is tiled to fill the plane of an image,
thereby forming a halftone screen of threshold values. At a given
pixel, if the required color tone level is darker than the given
halftone threshold level, a color spot is printed at that specified
pixel. Otherwise the color spot is not printed. The output of the
screening process is a binary pattern of multiple small "dots,"
which are regularly spaced as is determined by the size, shape, and
tiling of the halftone cell. In other words, the screening output,
as a two-dimensionally repeated pattern, possesses two fundamental
spatial frequencies, which are completely defined by the geometry
of the halftone screen.
It is understood in the art that the distribution of printed pixels
depends on the design of the halftone screen. For clustered-dot
halftone screens, all printed pixels formed using a single halftone
cell typically group into one or more clusters. If a halftone cell
only generates a single cluster, it is referred to as a single-dot
halftone or single-dot halftone screen. Alternatively, halftone
screens may be dual-dot, tri-dot, quad-dot, or the like.
While halftoning is often described in terms of halftone dots, it
should be appreciated by those skilled in the art that idealized
halftone dots can possess a variety of shapes that include
rectangles, squares, lines, circles, ellipses, "plus signs,"
X-shapes, pinwheels, and pincushions, and actual printed dots can
possess distortions and fragmentation of those idealized shapes
introduced by digitization and the physical printing process.
Various digital halftone screens having different shapes and angles
are described in U.S. Pat. No. 4,149,194, the disclosure of which
is incorporated herein by reference in its entirety.
A common problem that arises in digital color halftoning is the
manifestation of moire patterns. Moire patterns are undesirable
interference patterns that occur when two or more color halftone
separations are printed over each other. Since color mixing during
the printing process is a non-linear process, frequency components
other than the fundamental frequencies and harmonics of the
individual color halftone separations can occur in the final
printout. For example, if an identical halftone screen is used for
two color separations, theoretically, there should be no moire
patterns. However, any slight misalignment between the two color
halftone separations occurring from an angular difference and/or a
scalar difference will result in two slightly different fundamental
frequency vectors. Due to nonlinear color mixing the difference in
frequency vectors produces a beat frequency which will be visibly
evident as a very pronounced moire interference pattern in the
output. To avoid, for example, two-color moire patterns due to
misalignment, or for other reasons, different halftone screens are
commonly used for different color separations, where the
fundamental frequency vectors of the different halftone screens are
separated by relatively large angles. Therefore, the frequency
difference between any two fundamental frequencies of the different
screens will be large enough so that no visibly objectionable moire
patterns are produced.
In selecting different halftone screens, for example for three
color separations, it is desirable to avoid any two-color moire as
well as any three-color moire. It will be appreciated that in the
traditional printing industry that three halftone screens, which
can be constructed by halftone cells that are square in shape and
identical, can be placed at 15.degree., 45.degree., and 75.degree.,
respectively, from a point and axis of origin, to provide the
classical three-color moire-free solution.
However, for digital halftoning, the freedom to rotate a halftone
screen is limited by the raster structure, which defines the
position of each pixel. Since) tan(15.degree. and)tan(75.degree.
are irrational numbers, rotating a halftone screen to 15.degree. or
75.degree. cannot be exactly implemented in digital halftoning. To
this end, some methods have been proposed to provide approximate
instead of exact moire-free solutions. For example, in U.S. Pat.
Nos. 5,323,245 and 5,583,660, this problem is approached by using a
combination of two or more perpendicular, unequal frequency screen
patterns and non-perpendicular, equal frequency non-conventional
screen patterns. However, all these approximate solutions result in
some halftone dots having centers that do not lie directly on
addressable points, or on the pixel positions defined by the raster
structure. Therefore, the shape and center location varies from one
halftone dot to another. Consequently, additional interference or
moire between the screen frequencies and the raster frequency can
occur. In another approach, U.S. Pat. No. 5,371,612 discloses a
moire prevention method to determine screen angles and sizes that
is usable solely for square-shaped, halftone screens.
Customers who use clustered dot halftoning such as laser printing
or offset printing may use halftone geometries. However, existing
halftone geometries are constrained, capable of providing only
limited options with respect to halftone angle and frequency. Given
such constraints, it is difficult to satisfy multiple system
requirements, e.g. a requirement that halftones be moire-free, not
beat with multiple frequency components from the raster output
system, screen visibility, and be free of halftone artifacts. It is
also desirable to avoid the use of 0.degree. screens, which give
rise to multiple image processing issues. Many attempts have been
made to solve these issues, however none have produced a complete
solution.
U.S. Pat. No. 7,898,692 to Wang and voce, entitled "Rosette
Printing with up to Five Colors" produces moire-free color halftone
printing with up to five color image separations. It also uses a
plurality of non-orthogonal halftone screens, defines a first and
second color halftone screen fundamental frequency vector for each
of three halftone screens which produces moire-free rosettes, and
defines a fourth color halftone with the first fundamental vector
of the fourth screen shares a fundamental frequency vector with one
of said three halftone screens and a second fundamental frequency
vector of the fourth screen shares a fundamental frequency vector
with a different one of said three color halftone screens. Further,
it defines a fifth color halftone screen where a first fundamental
vector of the fifth screen shares a fundamental frequency vector
with one of the three halftone screens and a second fundamental
frequency vector of the fifth screen shares a fundamental frequency
vector with a different one of the three color halftone screens.
None of the fundamental frequency vectors of the fifth screen are
equal to either of the fundamental frequency vectors of the fourth
screen. The disclosure of U.S. Pat. No. 7,898,692 is hereby
incorporated by reference in its entirety.
U.S. Pat. No. 7,675,651 to Wang and Lace, entitled "Moire-free
color halftone configuration employing common frequency vectors",
produces moire-free color halftone printing of up to four color
image separations by using a plurality of non-orthogonal halftone
screens to produce moire-free prints that form uniform periodic
rosettes. It uses a first and second color halftone screen
fundamental frequency vector designed for each of three halftone
screens such that the halftone screen set output forms uniform
hexagonal rosettes. It also defines a fourth color halftone screen
where a first fundamental vector of the fourth screen shares a
fundamental frequency vector with one of the three halftone
screens. It also defines a second fundamental frequency vector of
the fourth screen that shares a fundamental frequency vector with a
different one of said three color halftone screens. The disclosure
of U.S. Pat. No. 7,675,651 is hereby incorporated by reference in
its entirety.
U.S. Pat. No. 7,480,076, to Wang, entitled "Moire-Free Color
Halftone Configuration", is directed to moire-free color halftone
configurations for clustered dots. Unlike conventional methods, the
disclosed method produces periodic hexagon rosettes of identical
shapes. These exemplary hexagon rosettes have three fundamental
spatial frequencies exactly equal to half of the fundamental
frequency of the three halftone screens. The resultant halftone
outputs are truly moire-free, as all the fundamentals and harmonic
frequencies are multiples of, and thus higher in frequency than,
the rosette fundamental frequency. The disclosure of U.S. Pat. No.
7,480,076 is hereby incorporated by reference in its entirety.
U.S. Pat. No. 6,798,539 to Wang, Fan, and Wen, entitled "Method for
Moire-Free Color Halftoning Using Non-Orthogonal Cluster Screens",
is directed to the use of single-celled, non-orthogonal
clustered-dot screens to satisfy the moire-free conditions for
color halftoning. The disclosure also provides methods that combine
single-cell non-orthogonal clustered-dot screens and line screens
for moire-free color halftoning. Particularly, the selection of
these single-cell halftone screens is determined by satisfying
moire-free conditions provided in the respective spatial or
frequency equations. The disclosure of U.S. Pat. No. 6,798,539 is
hereby incorporated by reference in its entirety.
U.S. Pat. No. 7,679,787 to Wang and Lace, entitled "N-Color
Printing with Hexagonal Rosettes", produces moire-free enhanced
color halftone printing of color image separations for an arbitrary
number of colorants. It uses a plurality of halftone screens to
produce outputs that are moire free and form hexagonal periodic
rosettes. A large number of screens can be used for enhanced
printing applications, such as printing with high-fidelity
colorants, light colorants, or special colorants, such as white,
metallics and fluorescents. It defines rosette fundamental
frequency vectors V.sub.R1, V.sub.R2 that satisfy a length and sum
requirement to meet visual acceptability standards according to
|V.sub.R1|>f.sub.min, |V.sub.R2|>f.sub.min, and
|V.sub.R1.+-.V.sub.R2|>f.sub.min. It also defines N halftone
screens for colorants i=1, N, respectively possessing first and
second frequency vectors (V.sub.i1, V.sub.i2), where no two screens
possess identical fundamental frequency vector pairs. It then
selects fundamental frequency vectors for the N halftone screens
according to (V.sub.i1,
V.sub.i2)=(m.sub.i1V.sub.R1+m.sub.i2V.sub.R2,
n.sub.i1V.sub.R1+n.sub.i2V.sub.R2) for integer m's and n's, where
at least one fundamental frequency vector or its conjugate must
also satisfy one of the following: V.sub.ik=V.sub.R1,
V.sub.ik=V.sub.R2, and "|V.sub.ik|>2 max [|V.sub.R1|,
|V.sub.R2|]. The disclosure of U.S. Pat. No. 7,679,787 is hereby
incorporated by reference in its entirety. What is needed in the
art is a versatile adjustment of a moire free halftone set such as
that angles and frequencies may be optimized for a given imaging
printing system.
Incorporation By Reference
S. Wang, Z. Fan and Z. Wen, "Non-Orthogonal Halftone Screens,"
Proc. NIP18: International Conference on Digital Printing
Technologies, pages 578-584, 2002.
R. Ulichney, "Digital Halftoning," The MIT Press, pages 117-126,
1988.
M. Turbek, S. Weed, T. Cholewo, B. Damon, M. Lhamon, "Comparison of
Hexagonal and Square Dot Centers for EP Halftones," PICS 2000,
pages 321-325.
BRIEF DESCRIPTION
In some illustrative embodiments disclosed as illustrative examples
herein, a computer-implemented method for generating halftone
screens for moire-free color halftoning, comprises identifying a
first color halftone screen having halftone cells defined by a
first fundamental frequency vector (V.sub.1) and a second
fundamental frequency vector (V.sub.2), and selecting at least one
shearing value (s) representative of an offset in at least one of a
fast scan direction and a slow scan direction. The method further
comprises shearing at least one of the first fundamental frequency
vector (V.sub.1) and the second fundamental frequency vector
(V.sub.2) for the first halftone screen by application of the
shearing value (s) associated therewith, and generating a first
sheared color halftone screen in accordance with at least one of
the sheared first fundamental frequency vector (V.sub.1) and the
second fundamental frequency vector (V.sub.2) so as to avoid
moireimage effects.
In some illustrative embodiments disclosed as illustrative examples
herein, a computer-implemented method for generating a set of
sheared color halftone screens for moire-free color halftoning,
comprises identifying a first color halftone screen having halftone
cells defined by a first fundamental frequency vector (V.sub.c1)
and a second fundamental frequency vector (V.sub.c2), a second
color halftone screen defined by a first fundamental frequency
vector (V.sub.m1) and a second fundamental frequency vector
(V.sub.m2), and a third color halftone screen defined by a first
fundamental vector (V.sub.k1) and a second fundamental frequency
vector (V.sub.k2). The method also comprises selecting a shearing
value (s.sub.c, s.sub.m, s.sub.k) corresponding to an offset in at
least one of a fast scan direction and a slow scan direction for
each of the first, second, and third color halftone screens, and
shearing at least one of the first fundamental frequency vectors
(V.sub.c1, V.sub.m1, V.sub.k1) and the second fundamental frequency
vectors (V.sub.c2, V.sub.m2, V.sub.k2) for each of the first,
second, and third halftone screens by application of the shearing
values (s.sub.c, s.sub.m, s.sub.k) corresponding thereto. The
method further comprises generating a set of sheared color halftone
screens in accordance with at least one of the sheared first
fundamental frequency vectors (V.sub.c1, V.sub.m1, V.sub.k1) and
the second fundamental frequency vectors (V.sub.c2, V.sub.m2,
V.sub.k2) so as to avoid moire image effects, and outputting the
set of sheared color halftone screens.
In some illustrative embodiments disclosed as illustrative examples
herein, a system that facilitates generating halftone screens for
moire-free color halftoning comprises a processor. The processor is
operable to identify a first color halftone screen having halftone
cells defined by a first fundamental frequency vector (V.sub.1) and
a second fundamental frequency vector (V.sub.2), and to select at
least one shearing value (s) representative of an offset in at
least one of a fast scan direction and a slow scan direction. The
processor is further operable to shear at least one of the first
fundamental frequency vector (V.sub.1) and the second fundamental
frequency vector (V.sub.2) for the first halftone screen by
application of the shearing value (s) associated therewith, and to
generate a first sheared color halftone screen in accordance with
at least one of the sheared first fundamental frequency vector
(V.sub.1) and the second fundamental frequency vector (V.sub.2) so
as to avoid moire image effects. The system further comprises a
printer that prints a halftone image using the first sheared color
halftone screen.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a method of generating halftone screens for
moire-free color halftoning using frequency vector shearing in
accordance with one embodiment of the subject application.
FIG. 2 illustrates spatial vectors defining a nonorthogonal
halftone cell in accordance with one embodiment of the subject
application.
FIG. 3 illustrates two corresponding fundamental frequencies of the
Fourier transform function in accordance with one embodiment of the
subject application.
FIG. 4 illustrates alternative start-of-scanline capabilities in
accordance with one embodiment of the subject application.
FIG. 5 illustrates a 4-phase clock employed in the prototype VCSEL
ROS in accordance with one embodiment of the subject
application.
FIG. 6 illustrates a halftone pattern with visible moire s by a
normal screen setting in accordance with one embodiment of the
subject application.
FIG. 7 illustrates a digital simulated halftone output of a
three-color moire-free geometry with one shearing in accordance
with one embodiment of the subject application.
FIG. 8 illustrates a close-up view of the halftone screens
associated with FIG. 6 and FIG. 7 in accordance with one embodiment
of the subject application.
FIG. 9 illustrates a close-up view of the cyan halftone screens
associated with FIG. 8 in accordance with one embodiment of the
subject application.
FIG. 10 illustrates a close-up view of the magenta halftone screens
associated with FIG. 8 in accordance with one embodiment of the
subject application.
FIG. 11 illustrates a computer system that is capable of
implementation to facilitate generation of halftone screens for
moire-free color halftoning using frequency vector shearing in
accordance with one embodiment of the subject application.
FIG. 12 illustrates a system that facilitates generation of
halftone screens for moire-free color halftoning using frequency
vector shearing in accordance with one embodiment of the subject
application.
DETAILED DESCRIPTION
The present disclosure and embodiments described herein provide
halftoning methods and halftoning configurations using modified
color halftone screens, or more precisely, sheared color halftone
screens for moire-free color printing.
The subject application presents a moire-free halftone geometry
which uses shearing of the frequency vectors in the fast scanning
and/or slow scanning direction. One embodiment that may achieve
such shearing is to apply a line-to-line offset in the
start-of-scan signal for the raster line. Current vertical-cavity
surface-emitting laser raster output scanner electronics use a
4-phase clock to enable a 9600 spi resolution (4.times.2400 spi)
for start-of-scan. It will be understood by those skilled in the
art that while such a resolution is currently sufficient for
certain embodiments of the subject application, other, higher
resolutions may be needed for future electronics, so as to provide
an analog tuning of the shear of a screen or screen set. In
addition to the enabling of new geometries via shearing, as
explained in greater detail below, the shearing parameter, or
offset, may be used so as to adjust given halftone sets to have
shear that may be preferred for particular images, e.g. tinted
text, tinted italic text, and angled graphics.
Turning now to FIG. 1, there is shown a method 100 of generating
halftone screens for moire-free color halftoning using frequency
vector shearing, in accordance with the example embodiments
described herein. At step 102, a halftone screen is identified
comprising a plurality of halftone cells. The skilled artisan will
appreciate that the color halftoning operations employ multiple
color screens to achieve the desired appearance of a particular
color to the viewer. Accordingly, the methodology 100 of FIG. 1
identifies one of these color halftone screens, Cyan, Magenta,
Yellow, blacK, commonly referenced as CMYK. Those skilled in the
art will appreciate that other colorants are also capable of being
employed and adapted in accordance with the subject
application.
In accordance with one example embodiment of the subject
application, the halftone screen identified at step 102 may
correspond to a first screen from a particular set of halftone
screens which a user desires to use, e.g. the set provides
desirable flesh tone reproduction, landscape photographic
reproduction, provides clear italic typeset, textured fabric
photographic reproduction, or the like. However, past experience or
attempted usage indicates that such a set of screens in the
particular application, i.e. photograph, magazine printing,
italics, or the like, results in beating, interference with the
subject matter, or moire patterns appearing in the output
results.
It will be appreciated by those skilled in the art that the
methodology 100 illustrated in FIG. 1 is capable of being
implemented by a computer system 150, which comprises at least a
processor (such as the processor 1102 of FIG. 11) that executes,
and a memory (such as the memory 1104 of FIG. 11) that stores,
computer-executable instructions for providing the various
functions, calculations, selections, and the like, described
herein.
In accordance with one embodiment of the subject application, the
computer system 150 is capable of being employed as one possible
hardware configuration to support the systems and methods described
herein. The skilled artisan will further appreciate that although
illustrated as a standalone device, any suitable computing
environment is capable of being employed in accordance with the
subject application. For example, computing architectures
including, but not limited to, multiprocessor, distributed,
client/server, tablet, mainframe, supercomputer, digital and analog
can be employed in accordance with the one embodiment of the
subject application.
The computer 150 can include a processing unit (see, e.g. FIG. 11),
a system memory (see, e.g. FIG. 11), and a system bus (such as the
bus 1112 of FIG. 11) that couples various system components
including the system memory to the processing unit. The processing
unit can be any of various commercially available processors. Dual
microprocessors and other multi-processor architectures also can be
used as the processing unit.
The computer 150 typically includes at least some form of computer
readable media. Computer readable media can be any available media
that can be accessed by the computer. For example, and without
limitation, computer readable media may comprise computer storage
media and communication media. Computer storage media includes
volatile and nonvolatile, removable and non-removable media
implemented in any method or technology for storage of information
such as computer readable instructions, data structures, program
modules or other data.
Communication media typically embodies computer readable
instructions, data structures, program modules or other data in a
modulated data signal such as a carrier wave or other transport
mechanism and includes any information delivery media. The term
"modulated data signal" means a signal that has one or more of its
characteristics set or changed in such a manner as to encode
information in the signal. Communication media includes, for
example, and without limitation, BLUETOOTH, WiMax, 802.11a,
802.11b, 802.11g, 802.11(x), a proprietary communications channel,
infrared, optical, the public switched telephone network, or any
suitable wireless data transmission system, or wired communications
known in the art. Combinations of any of the above can also be
included within the scope of computer readable media.
A user may enter commands and information into the computer through
an input device (see, e.g. FIG. 11) such as a keyboard, a pointing
device, such as a mouse, stylus, voice input, or graphical tablet.
The computer 150 is capable of operating in a networked environment
using logical and/or physical connections to one or more remote
computers, such as a remote computer(s). The logical connections
depicted include a local area network (LAN) and a wide area network
(WAN). Such networking environments are commonplace in offices,
enterprise-wide computer networks, intranets and the Internet.
Additional functioning of the computer 150 with respect to the
example computer system 1100 of FIG. 11, discussed in greater
detail below.
Returning to step 102, an example non-orthogonal halftone cell of
the identified halftone screen is illustrated in FIG. 2. As shown
in FIG. 2, a general non-orthogonal halftone cell 200 is defined in
the spatial domain. It will be understood by those skilled in the
art that the cell 200 is depicted as a parallelogram for example
purposes only, and the subject application is capable of
implementation on a variety of halftone cell shapes.
The general non-orthogonal halftone cell 200 is plotted on a
horizontal axis 202, and a vertical axis 204, enabling a general
nonorthogonal halftone screen to be specified by two vectors,
v.sub.1(x.sub.1, y.sub.1) 206 and v.sub.2(x.sub.2, y.sub.2) 208.
Each vector has a perpendicular such that v.sub.1 206 has a
perpendicular h.sub.1 214 and a parallel line 210, while v.sub.2
208 has a perpendicular h.sub.2 216 and a parallel 212.
It will be appreciated by those skilled in the art that by using
Fourier transforms, it is possible to represent the spatial vectors
206 and 208 in the frequency domain. Therefore, at step 104, a
first fundamental frequency vector (V.sub.1) and a second
fundamental frequency vector (V.sub.2) are identified that define
the halftone cell comprising the halftone screen. Accordingly, FIG.
3 illustrates such a frequency domain 300 representation of the
spatial vectors 206 and 208. Thus, the skilled artisan will
appreciate that the screen can be represented by two frequency
vectors, V.sub.1(f.sub.x1,f.sub.y1) 306 and
V.sub.2(f.sub.x2,f.sub.y2) 308, plotted along the f.sub.x-axis 302
and f.sub.y-axis 304. Similar to an orthogonal case, V.sub.1 306
and V.sub.2 308 are perpendicular to v.sub.1 206 and v.sub.2 208,
respectively. However, the moduli of the frequency vectors
|V.sub.1| 306 and |V.sub.2| 308 are not given by the reciprocals of
|v.sub.2| 206 and |v.sub.1| 208, as for the orthogonal screens.
Instead, |V.sub.1| 306 and |V.sub.2| 308 are equal to the
reciprocals of h.sub.1 214 and h.sub.2 216, which are the heights,
or the pitches shown by dot lines 406 of FIG. 4, as discussed in
greater detail below. Since the product,
|v.sub.1|*h.sub.1.quadrature.=.sub..quadrature.|v.sub.2|*h.sub.2.quadratu-
re.=.sub..quadrature. A, is the area of the specified parallelogram
bounded by the sides 206, 208, 210, 212, the moduli of the
frequency vectors V.sub.1 306 and V.sub.2 308 may be given by:
.times..times. ##EQU00001##
where A is given by the absolute value of the cross product of the
two spatial domain vectors 206 and 208, v.sub.1.times.v.sub.2.
Therefore, A may be expressed as a function of the spatial
coordinates x.sub.1, y.sub.1, x.sub.2, and y.sub.2, by:
A=|x.sub.1y.sub.2-x.sub.2y.sub.1|. (2)
Since the spatial vector v.sub.1(x.sub.1, y.sub.1) and the
frequency vector V.sub.1(f.sub.x1, f.sub.y1) are perpendicular to
each other, and that v.sub.2(x.sub.2, y.sub.2) and
V.sub.2(f.sub.x2, f.sub.y2) are perpendicular, from Eqs. (1a) and
(1b), the frequency vectors V.sub.1 and V.sub.2 may be decomposed
into their scalar components as:
.times..times..times..times..times. ##EQU00002##
Therefore, Eqs. (3a)-(3d) express the
frequency-to-spatial-component relationship for the cell 200
defined by the spatial vectors v.sub.1 206 and v.sub.2 208.
Although, in general, the frequency components, f.sub.x1, f.sub.y1,
f.sub.x2, and f.sub.y2 are real numbers, they are also rational
numbers completely defined by the four integer coordinate values,
x.sub.1, y.sub.1, x.sub.2, and y.sub.2. Since Eqs. (3a)-(3d)
describe a corresponding "mapping" of the frequency components to
the spatial components, it will be appreciated by those skilled in
the art that any analysis of the moire-free conditions in the
frequency domain are capable of easy translation into a spatial
domain specification. It will further be appreciated that, while
the above equations are developed in relation to the non-orthogonal
halftone cell 200 having a parallelogram-like shape, it is apparent
that the above equations may suitably describe other
non-parallelogram shaped cells, for example, squares, rectangles,
triangles, ellipses, etc., in accordance with the subject
application.
Continuing with the example depicted in FIGS. 2 and 3, a
parallelogram may be specified by the two frequency vectors V.sub.1
306 and V.sub.2 308, such that the frequency domain parallelogram
is a rotated and scaled version of the parallelogram halftone 200
screen with a 90.degree. rotation and 1/A scaling. Thus, if the
following condition: x.sub.1/y.sub.1=-y.sub.2/x.sub.2, (4) is
satisfied, the general parallelogram becomes a rectangle.
Furthermore, if: x.sub.1=.+-.y.sub.2, and (5a) y.sub.1=.-+.x.sub.2,
(5b) then the parallelogram becomes a square.
According to one particular application of the subject application,
the non-orthogonal halftone screens are capable of providing exact
solutions for moire-free color halftoning. For example, in color
printing, the undesirable moire can result from the superposition
of the halftone screens of the different process colorants, e.g.
cyan, magenta, yellow, black, and interaction between the screens.
The interactions can be due to "unwanted optical absorption" or
physical interaction, such as development suppression of one
colorant by another colorant. Using Fourier analysis applied to
halftone screens, the result caused by superposition of two
different colorants may be expressed as their frequency-vector
difference, V.sub.cm=V.sub.c.+-.V.sub.m, where V.sub.c and V.sub.m
are two frequency components from two different colorants, e.g.,
cyan and magenta, and V.sub.cm is the difference vector. Since each
Fourier component has a corresponding conjugate, i.e. there is
always a frequency vector -V.sub.c that represents the conjugate
component of V.sub.c, the sign definition of frequency vectors is
rather arbitrary. For each parallelogram screen, there are two
fundamental frequency vectors, therefore, the color mixing of two
screens for two different colorants yields four difference vectors.
If any one of these difference vectors is much shorter than the
cut-off frequency of the sensitivity function of the human visual
system and not very close to zero, there is a possibility to have
two-color (ant) moire appearing on the halftone output at the
frequency represented by the corresponding difference vector. Given
that the common strategy to avoid any two-color moire is to make
sure that no two-color difference vector will be too small, the
two-color moire-free condition can be summarized by:
|V.sub.c.+-.V.sub.m|>V.sub.min, (6) where V.sub.c=V.sub.c1,
-V.sub.c1, V.sub.c2, V.sub.c2; V.sub.m=V.sub.m1, -V.sub.m1,
V.sub.m2, -V.sub.m2, and V.sub.min is a frequency limit set at
somewhere 50-70 line-per-inch for just-noticeable moire s.
The skilled artisan will appreciate that the most troublesome moire
is the three-colorant moire, usually appearing as the
cyan-magenta-black moire in prints produced by CMYK four-color
printers. As an extension of the two-color case, the three-color
moire-free condition can be summarized by:
|V.sub.c.+-.V.sub.m.+-.V.sub.k|>V.sub.min, (7) where
V.sub.c=V.sub.c1, -V.sub.c1, V.sub.c2, -V.sub.c2; V.sub.m=V.sub.m1,
-V.sub.m1, V.sub.m2, -V.sub.m2; V.sub.k=V.sub.k1, -V.sub.k1,
V.sub.k2,-V.sub.k2, and V.sub.min is set similar to the two-color
case. Since there are altogether thirty-two different combinations
of different colorant components, practically, to make all
three-color difference vectors, as well as all two-color difference
vectors, large enough to avoid any color moire is very difficult
unless the halftone screens have very high fundamentals
frequencies, e.g. higher than 200 line-per-inch. Another aspect of
the moire-free condition is to make two of the three-color
difference vectors null while keeping the remaining differences
large. It should be appreciated that the two frequency vectors
V.sub.1 and -V.sub.1 are exchangeable. Further, it should be
appreciated that the arbitrary indices 1 and 2 may be exchanged
between the two frequency vectors V.sub.1 and V.sub.2 in each color
separation. Thus, given that both the signs and the indices of the
frequency vectors are defined somewhat arbitrarily, without losing
the generality, the three-color moire-free condition may be
specified by the following two vector equations:
V.sub.c.sub.1+V.sub.m.sub.1+V.sub.k.sub.1=0, and (8a)
V.sub.c.sub.2+V.sub.m.sub.2+V.sub.k.sub.2=0. (8b)
It is not difficult to prove that, once the two equations, (8a) and
(8b), are satisfied, the remaining combinations of three color
components are equal to a linear combination of higher-order
harmonics from two colors. In most practical applications, this
will satisfy the inequality set forth in Eq. (7).
Using the scalar components of the frequency representation and
Eqs. (3a)-(3d), and the above moire-free conditions, a scalar
representation of Eqs. (8a)-(8b) may be translated into scalar
equations (9a)-(9d) as follows:
.times..times..times..times..times..times. ##EQU00003##
It should be appreciated that, if the respective spatial coordinate
values x.sub.1, x.sub.2, and y.sub.1, y.sub.2 are integer values,
the four equations (9a)-(9d) may be converted to:
A.sub.mA.sub.kx.sub.c.sub.1+A.sub.cA.sub.kx.sub.m.sub.1+A.sub.cA.sub.mx.s-
ub.k.sub.1=0 (10a)
A.sub.mA.sub.ky.sub.c.sub.1+A.sub.cA.sub.ky.sub.m.sub.1=A.sub.cA.sub.my.s-
ub.k.sub.1=0 (10b)
A.sub.mA.sub.kX.sub.c.sub.2+A.sub.cA.sub.kx.sub.m.sub.2+A.sub.cA.sub.mx.s-
ub.k.sub.2=0, and (10c)
A.sub.mA.sub.ky.sub.c.sub.2+A.sub.cA.sub.ky.sub.m.sub.2+A.sub.cA.sub.my.s-
ub.k.sub.2=0. (10d)
Using Eq. (2) and the three areas A.sub.c, A.sub.m and A.sub.k in
Eqs. (10a)-(10d), it follows, therefore that the values of the
parallelogram area are given by, or rewritten as:
A.sub.c=|x.sub.c.sub.1y.sub.c.sub.2-x.sub.c.sub.2y.sub.c.sub.1|,
(11a)
A.sub.m=|x.sub.m.sub.1y.sub.m.sub.2-x.sub.m.sub.2y.sub.m.sub.1|,
(11b)
A.sub.k=|x.sub.k.sub.1y.sub.k.sub.2-x.sub.k.sub.2y.sub.k.sub.1|
(11c) Alternatively, two vector equations in spatial vectors can be
derived from Eqs. (9a)-(9d), i.e.
.times..times. ##EQU00004##
The present disclosure and embodiments described herein include
methods and halftone configurations that provide moire-free
halftone geometries via shearing of the frequency vectors in one or
more directions, e.g. the fast scanning or slow scanning
directions. For example, a frequency vector extending from the
origin to a point in the first quadrant may be used as an example
of shearing, such that the endpoint of the vector in the first
quadrant is shifted a predetermined amount along a line parallel to
the x-axis, while the other end of the vector remains fixed at the
origin. Each point along that parallel line corresponds to a
different halftone in the spatial domain, such that the movement of
the vector endpoint would "shear" the halftone structures in the
spatial domain. Halftone designers consider many options to deliver
a screen with desirable characteristics, and often must settle for
less than desirable results. The present method presents a new
option with several beneficial properties compared to conventional
screens. In addition to enabling new geometries via shearing,
various shearing parameters can be made available to knowledgeable
users so they can adjust given halftone sets to have shear that may
be preferred for particular images, such as tinted text, tinted
italic text, angled graphics, and the like.
As discussed in greater detail below, one aspect of the subject
application enables halftone geometries of interest by shearing in
the fast-scanning direction via the application of a line-to-line
offset in the start-of-scan signal for each raster line produced by
a laser scanner. The offsets of interest would tend to be linearly
increasing with raster line number.
It is generally desired to use halftones that are moire-free, and
not beat with multiple frequency components from the raster output
scanner, e.g. interference of two frequency components that cause
visibility issues. Previous attempts at beat avoidance resulted in
custom designed halftone screen sets that comprise a particular
geometry that avoids raster output scanner beating by setting the
frequency components of that geometry as harmonics of the raster
output scanner. Unfortunately, those screen sets contain a
0.degree./90.degree. screen, which users find objectionable and
that have a negative interaction with calibration efforts.
More generally, the present application improves digital halftoning
by increasing the number of screen frequencies and/or angles that
are realizable; while also increasing the accuracy in approximating
an irrational screen. The skilled artisan will appreciate that the
subject application, via the screen frequencies and angles
increase, provide freedom in meeting conflicting requirements such
as moire-free, beating avoidance, screen visibility, and halftone
artifacts. It will also be appreciated by those skilled in the art
that the increase in accuracy for irrational screen approximation
is advantageous where accuracy for approximating certain halftone
frequencies (e.g. 75.degree.) is essential.
Returning to FIG. 1, at step 106, a shearing value (s) for
application to the fundamental frequency vectors (V.sub.1,V.sub.2)
306 and 308 is selected for offsetting the example halftone cell
200 of FIG. 2. It will be appreciated by those skilled in the art
that the subject application provides for multiple means for
selecting a suitable shearing value (s), depending upon the desired
output halftone screen. Such values (s) may be selected by, for
example and without limitation, pixel clock manipulation, distance
in the x-direction and/or y-direction, angle, specific pixel
locations, and the like.
It will be appreciated that offsetting the pixel clock for
different scan lines may enable an improved halftone image. FIG. 4
illustrates an alternating start-of-scan diagram 400 as scan lines
of individual pixels 402.
Alternating start-of-scan offsets for every other line of pixels
402 to form a hexagonal-like pixel grid 404 thus generates
hexagonal halftone cells 406 resulting in scan lines offset by
alternating between two phases of a pixel clock. [Have removed the
confusing language. We want to leave enough in here, including FIG.
4, to provide a bit of basis for what is stated later on in the
application, and if need be, during prosecution]
FIG. 5 illustrates a 4-phase clock employed in one example
vertical-cavity surface-emitting laser raster output scanner, such
as the associated control module (not shown). A scan line is
capable of starting on any pixel boundary of any of the phases 500.
The use of 4 phases at 2400 spi provides a resolution of 9600 spi
for start-of-scan. Those skilled in the art will appreciate that
current FPGA's are available with the capabilities to generate 8
and 16 phases in the event that such additional resolution is
needed. Thus it will be appreciated by the skilled artisan that the
selection of 4 phases is for example purposes only. In accordance
with one embodiment of the subject application, start-of-scan
phases and full pixel offset are capable of being varied from scan
line to scan line. It will be appreciated by those skilled in the
art that while this capability may be used for registration
purposes, the subject application will use it to achieve certain
halftone geometries.
As illustrated in FIG. 5, a phase begins at a 0.degree. angle
raster line 502. The next raster line is at a 90.degree. phase 504.
The next raster is at a 180.degree. phase 506. The next raster line
is at a 270.degree. phase 508. Each raster line has a series of
sections labeled D0, 510, D1, 512, D2, 514, D3, 516, and D4, 518.
The arrangement is slightly askew such that the raster line
segments D0 to D4 do not overlap, i.e. align directly in the slow
scan direction, instead stacking above and below each other in a
brick wall type arrangement. Thus, the raster line in the
90.degree. phase line contains a D1 segment that overlaps with the
D0 segment in 180.degree. and 270.degree., phase lines while also
overlapping with the D2 segment in the 0.degree. phase line. Usage
of the output of the phase, or pixel, clock described with respect
to FIG. 5 will be more readily understood in conjunction with the
methodologies described below.
In accordance with one embodiment of the subject application,
shearing is performed in the slow-scanning direction, by one or
more means that are capable of being used for scan line alignment.
For example, in a raster output scanner imaging system, the raster
output scanner itself or an optical component, such as a mirror,
can be tilted to shear the scan line in the slow scan direction.
That is, the entire raster output scanner is moved with a stepper
motor to a target angle, or mirrors in the optical system are
tilted to align the scan line. In image bar imaging systems, a
timing offset can be applied to each column of pixels or the bar
can be tilted to align the scan line, i.e. controllable pixel
column timing for slow-scan raster line adjustment.
According to one embodiment of the subject application, shearing is
performed by varying the starting phase of scanning. That is, the
selection of a shear (s) is suitably defined in terms of linear
horizontal displacement as a function of y. Accordingly, described
hereinafter is an example of shearing along one direction so as to
enable new halftone geometries. In such an example embodiment, the
shearing corresponds to an offset in the fast-scan direction. The
skilled artisan will appreciate that the following analysis is also
applicable to shearing in the slow-scan direction.
The effect of linearly varying the phase at start-of-scan is a
horizontal linear shearing of the two-dimensional image, i.e.
horizontal shearing of the cell 200 of FIG. 2. Thus, the shear (s)
may be designated as (.epsilon.), which represents a shearing
offset in the horizontal direction, which can be considered a
linear coefficient between the horizontal shift (.DELTA.x) and the
vertical distance, i.e. the slow-scan position (y) of the raster
line, such that: .DELTA.x=.epsilon.y (13) The actual output
location of a point specified by the nominal coordinate (x.sub.o,
y.sub.o) thereby becomes: x=x.sub.o+.epsilon.y.sub.o, and (14a)
y=y.sub.o (14b)
It will be appreciated by those skilled in the art that while
reference is made to selecting a shearing value (s) as (.epsilon.)
for representation of a shift in the horizontal direction, (s) is
capable of implementation in the embodiments described below as an
angular offset of the frequency vectors (V.sub.1, V.sub.2), as a
periodic or line-to-line offset in the start-of-scan signal for a
raster line, a shift in the vertical direction (e.g. .DELTA.y), a
periodic displacement as a function of y, or the like. According to
one example embodiment, the shearing offset (s) is selected in
accordance with a desired modification to a halftone geometry, as
will be appreciated by those skilled in the art. For example, the
value (s) may be selected so as to rotate one or more halftone
screens, so as to avoid a moire-effect, as set forth in greater
detail below.
For example purposes, (s) is specified hereinafter in terms of
linear horizontal displacement as a function of y, designated as
the shearing value (.epsilon.). Returning to FIG. 1, once the
shearing value (.epsilon.) has been selected at step 106, at least
the first fundamental frequency vector (V.sub.1) 306 or the second
fundamental frequency vector (V.sub.2) 308 is sheared accordingly
at step 108. In accordance with the discussions above, the spatial
vectors (v.sub.1 206 and v.sub.2 208) of the halftone written with
start-of-scan shear become: v.sub.1(x.sub.1,
y.sub.1)=v.sub.1(x.sub.o1+.epsilon.y.sub.o1, y.sub.o1),and (15a)
v.sub.2(x.sub.2, y.sub.2)=v.sub.2(x.sub.o2+.epsilon.y.sub.o2,
y.sub.o2). (15b) The skilled artisan will therefore appreciate that
the area A.sub.o specified by the nominal vectors v.sub.1(x.sub.o1,
y.sub.o1) and v.sub.2(x.sub.o2, y.sub.o2) is equal to the area
specified by the sheared vectors A.sub..epsilon.o:
A.sub.o=|x.sub.o1y.sub.o2-x.sub.o2y.sub.o1| (16c)
A.sub..epsilon..sub.o=|(x.sub.o1+.epsilon.y.sub.o1)y.sub.o2-(x.sub.-
o2+.epsilon.y.sub.o2)y.sub.o1|=A.sub.o (16d)
Accordingly, the two frequency vectors V.sub.1(f.sub.x1,f.sub.y1)
and V.sub.2(f.sub.x2,f.sub.y2) of the sheared output can be written
as:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times. ##EQU00005##
At step 110, the computer 150 or other suitable computing device
modifies the moire-free conditions in accordance with the sheared
fundamental frequency vector(s) (V.sub.1,V.sub.2) 306 and/or 308 so
as to generate a sheared halftone screen corresponding thereto.
That is, the halftone cell 200 is suitably sheared based upon the
corresponding frequency vectors, so as to reduce moirepatterning.
At step 112, the sheared halftone screen is suitably output for use
in moire-free halftoning operations.
In accordance with a previously discussed example implementation,
e.g. a reproduction of a photograph in a magazine requiring flesh
tones, once a set of halftone screens is identified, an attempt to
output a reproduction of the image is produced. In the event that
moire patterns are observed, one of the halftone screens is
identified, and the fundamental frequency vectors corresponding to
a non-orthogonal halftone cell of that identified screen are
determined. A shearing value (s) is then selected. As set forth
above, this shearing value (s) may be (.epsilon.), i.e. a simple
change in the distance along the fast scanning direction as a
function of y (e.g. a few micron difference in placement), altering
the angle of the vector (from which a suitable (s) may be derived),
altering distances in both the fast and slow scanning direction,
utilizing a pixel clock, or the like. The fundamental frequency
vectors are then sheared accordingly, wherein the above-identified
moire-free conditions are satisfied and the first halftone screen
corresponding to the modified frequency vectors is output. The
skilled artisan will appreciate that the additional colorant
screens are then similarly reproduced (discussed below).
Furthermore, the skilled artisan will appreciate that calculation
of the various remaining offsets for the additional colorant
screens may be accomplished as set forth above, by satisfying the
moire-free conditions using the initially selected offset
(.epsilon.), and the like.
The methodology of FIG. 1 will be better understood in conjunction
with the example implementations discussed hereafter, with respect
to three-color moire-free geometries with shearing in one direction
and shearing in two directions. The skilled artisan will appreciate
that while the following examples illustrate usage of (.epsilon.),
linear horizontal displacement as a function of y, other shears (s)
may be selected an implemented (angle, periodic displacement as a
function of y, or the like) in accordance with the methods and
systems set forth herein.
Three-Color Moire-Free Geometry with One Shearing
Those skilled in the art will appreciate that application of the
shearing to three-color halftoning processes is also capable of
reducing moire patterning. Accordingly, by shearing only in the
fast scanning (e.g. x-direction) for cyan, magenta, and black
screens using corresponding offsets .epsilon..sub.c,
.epsilon..sub.m, .epsilon..sub.k, respectively, the three-color
moire-free condition of Eqs. (9a)-(9d) may be modified as
follows:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times..times..times..times..-
times..times..times..times..times..times..times. ##EQU00006##
EXAMPLE 1
The following example serves to illustrate that new geometries are
enabled by shearing of frequency vectors in accordance with one
embodiment of the subject application. Accordingly, in the
following example, it is shown that a screen set that is not
moire-free may be made moire-free by use of appropriate shearing
according to the subject application. Thus, the following nominal
screens are used: v.sub.col(4, 1), v.sub.co2(2, -4); v.sub.co1(-2,
-4), v.sub.mo2(-4, 1)and v.sub.ko1(-3, 3), v.sub.ko2(3, 3).
The corresponding screen areas specified by the above vectors are:
A.sub.co=18, A.sub.mo=18 and A.sub.ko=18.
FIG. 6 illustrates the digital simulation of the halftone output
600 corresponding to the nominal screens above. As shown in FIG. 6,
the halftone output of above screen set is not moire-free; instead,
CMK three-color moire s 602 and 604 are noticeable at both plus and
minus 45 degrees.
The subject application provides for multiple shearing
possibilities that are capable of yielding a moire-free screen set
from these nominal halftones. That is, multiple values of the
offsets .epsilon..sub.c, .epsilon..sub.m, and .epsilon..sub.k are
capable of being determined in accordance with the methodology of
the subject application, and the use of the following values is for
example purposes only. Therefore, in continuing with the preceding
example, one solution to overcome the noted moire issues uses the
following values: .epsilon..sub.c=1/5; .epsilon..sub.m=-1/5,
.epsilon..sub.k=0.
With these selected parameters, the sheared halftone output shows
the following spatial periodicities: v.sub.c1(4.2, 1),
v.sub.c2(1.2, -4), v.sub.m1(-1.2, -4), v.sub.m2(-4.2, 1), and
v.sub.k1(-3, 3), v.sub.k2(3, 3). In a 600.times.600 dot-per-inch
printer, the halftoning result obtains the corresponding halftone
frequencies; and the combination thereof is three-color moire-free.
V.sub.c1(-33.3, 140), V.sub.c2(133.3, 40), or V.sub.c1(143.9 lpi
@13.4.degree.), V.sub.c2(139.2 lpi @73.3.degree.), V.sub.m1(-133.3,
40), V.sub.m2(33.3, 140), or V.sub.m1(139.2 lpi @73.3.degree.),
V.sub.m2(143.9 lpi @-13.4.degree.), and V.sub.k1(100, 100),
V.sub.k2(-100, 100), or V.sub.k1(141.4 @-45.0.degree.),
V.sub.k2(141.4 lpi @45.0.degree.).
FIG. 7 illustrates a digital simulation of the halftone output 700
by the above example as a three-color moire-free geometry with one
shearing. It will be appreciated by those skilled in the art that
the simulated halftone output 700 contains no discernible moire
issues. It should also be noted that at both plus and minus 45
degrees, the CMK three-color moire s 602 and 604 are no longer
visible in the output 700.
FIG. 8 depicts an extreme close-up view of the halftone screens of
FIG. 6 (screen 800) and FIG. 7 (screen 802). As shown in FIG. 8,
the halftone cells of the non-sheared screen black 804, cyan 808,
and magenta 810 are aligned in such a manner as to give rise to the
moire pattern exhibited in FIG. 6. In accordance with application
of the methodologies described above, shearing is advantageously
performed using the shearing values (.epsilon..sub.c=1/5;
.epsilon..sub.m=-1/5, .epsilon..sub.k=0). That is, the black 804
cells are not offset (.epsilon..sub.k=0), depicted on the screen
802 as the black cell 806. The cyan 808 cells are offset by
.epsilon..sub.c=1/5, which result in the cyan cells 810 of the
screen 802. Additionally, the magenta 812 cells are offset by
.epsilon..sub.m=-1/5, which result in the magenta cells 814 of the
screen 802. The skilled artisan will appreciate that subtle
movement of the aforementioned cells from no shearing (800) to
sheared (802) is visible in FIG. 8.
FIG. 9 illustrates only the cyan halftone screen corresponding to
the set of screens in FIG. 8. The first screen 900 represents an
unsheared, i.e. original cyan halftone screen corresponding to the
moire patterned image of FIG. 6. In contrast, the second screen 902
represents the sheared cyan halftone screen corresponding to the
moire-free image of FIG. 7. It should be appreciated that the cells
904 and 906 clearly illustrate a difference, i.e. shear, resulting
from application of the selected shearing value by
.epsilon..sub.c=1/5. FIG. 10 illustrates only the magenta halftone
screen corresponding to the set of screens in FIG. 8. The first
screen 1000 represents an unsheared, i.e. original magenta halftone
screen corresponding to the moire patterned image of FIG. 6. In
contrast, the second screen 1002 represents the sheared cyan
halftone screen corresponding to the moire-free image of FIG. 7. It
should be appreciated that the cells 1004 and 1006 clearly
illustrate a difference, i.e. shear, resulting from application of
the selected shearing value by .epsilon..sub.m=-1/5.
Three-Color Moire-Free Geometry with Two Shearings
By shearing cyan, magenta and black halftone in the x-direction,
respectively by .epsilon..sub.xc, .epsilon..sub.xm,
.epsilon..sub.xk, and those halftones in the y-direction
respectively by, .epsilon..sub.yc, .epsilon..sub.ym,
.epsilon..sub.yk, the three-color moire-free condition expressed
above in Eqs. (9a)-(9d) may be modified as follows:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times..times..times..times..-
times..times..times..times..times..times..times..times..times..times..time-
s..times..times..times..times..times..times..times..times..times..times..t-
imes..times..times..times..times..times..times..times..times..times..times-
..times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times.
##EQU00007##
EXAMPLE 2
The following example serves to illustrate that new geometries are
enabled by shearing of frequency vectors in both the fast-scanning
and slow-scanning directions according to one embodiment of the
subject application. Accordingly, in the following example, it is
shown that a screen set that is not moire-free may be made
moire-free by use of appropriate shearing in both the x-direction
and the y-direction according to the subject application. Thus, the
following nominal screens are used: v.sub.co1(1, 4), v.sub.co2(4,
-1), v.sub.mo1(-4, -1), v.sub.mo2(-1, 4), and v.sub.ko1(-3, 3),
v.sub.ko2(3, 3).
The corresponding screen areas specified by the above vectors are:
A.sub.c0=17, A.sub.mo=17 and A.sub.ko=18.
As set forth above with respect to single direction shearing, it is
possible to produce a moire-free set from this nominal set using
different shearing parameters. The following set of values is for
example purposes only and as such, the skilled artisan will
appreciate that other values of suitable offsets may be determined
in accordance with the subject application. Accordingly, the
following offsets serve to illustrate the application of two
direction shearing: .epsilon..sub.xc= 1/30, .epsilon..sub.xm=-
1/30, .epsilon..sub.xk=0, .epsilon..sub.yc= 1/30.1,
.epsilon..sub.ym=- 1/30.1, and .epsilon..sub.yk=0.
Using the above-identified offset values, the nominal screen
values, and Eqs. (19a)-(19d), the actual halftone output shows the
following periodicities: v.sub.c1(1.1333, 3.9623), v.sub.c2(3.9667,
-1.1318); v.sub.m1(-3.9667, -1.1318), v.sub.m2(-1.1333, 3.9623) and
v.sub.k1(-3, 3), v.sub.k2(3, 3).
In a 600.times.600 dot-per-inch printer, the halftoning result
obtains the corresponding halftone frequencies and their
combination is three-color moirefree: V.sub.c1(-139.9, 40),
V.sub.c2(39.9, 140), or V.sub.c1(145.5 lpi @74.0.degree.),
V.sub.c2(145.6 lpi @-15.9.degree.), V.sub.m1(39.9, -140),
V.sub.m2(139.9, 140), or V.sub.m1(145.6 lpi @-15.9.degree.),
V.sub.m2(145.5 lpi @-74.0.degree.), and V.sub.k1(100, 100),
V.sub.k2(-100, 100), or V.sub.k1(141.4 lpi @-45.0.degree.),
V.sub.k2(141.4 lpi @45.0.degree.).
Turning now to FIG. 11, illustrated is a representative computer
system 1100 (depicted in FIG. 1 as the computer 150) that
facilitates generating halftone screens for moire-free color
halftoning in connection with one embodiment of the subject
application. The computer system 1100 includes a processor unit
1102 which is advantageously placed in data communication with
memory 1104, which may include, for example and without limitation,
non-volatile read only memory, volatile read only memory, random
access memory or a combination thereof, a display interface 1106, a
storage interface 1108, and a network interface 1110. In one
embodiment, interface to the foregoing modules is suitably
accomplished via a bus 1112. The processor 1102 executes, and the
memory 1104 stores computer-executable instructions for performing
the various functions, methods, steps, techniques, and the like,
described herein. The processor 1102 and memory 1104 may be
integral to each other or remote but operably coupled to each
other.
The memory 1104 suitably includes firmware, such as static data or
fixed instructions, such as BIOS, system functions, configuration
data, and other routines used for operation of the computer system
1100 via the processor 1102. The memory 1104 is further capable of
providing a storage area for data and instructions associated with
applications and data handling accomplished by the processor
1102.
The display interface 1106 receives data or instructions from other
components on the bus 1112, which data is specific to generating a
display to facilitate a user interface. The display interface 1106
suitably provides output to a display device 1118, suitably a video
display such as a monitor, LCD, plasma, or any other suitable
visual output device as will be appreciated by one of ordinary
skill in the art.
As will be appreciated by those skilled in the art, the storage
interface 1108 is configured to provide a mechanism for
non-volatile, bulk or long term storage of data or instructions in
the computer system 1100. The storage interface 1108 suitably uses
a storage mechanism, such as storage 1116, suitably comprised of a
disk, tape, CD, DVD, or other relatively higher capacity
addressable or serial storage medium.
The network interface 1110 suitably comprises a network interface
card, a wireless network interface, or the like. It will be
appreciated that by one of ordinary skill in the art that a
suitable network interface is comprised of both physical and
protocol layers and is suitably any wired system, such as Ethernet,
token ring, or any other wide area or local area network
communication system, or wireless system, such as Wi-Fi, WiMax, or
any other suitable wireless network system, as will be appreciated
by one of ordinary skill in the art. In the illustration, the
network interface 1110 connected to a physical network 1120,
suitably comprised of a local area network, wide area network, or a
combination thereof.
An input/output interface 1114 in data communication with the bus
1112 is suitably connected with input devices, such as a keyboard,
mouse, pointing device, touch screen inputs, or the like. In
addition, the input/output interface 1114 is further capable of
data output to a peripheral interface, such as a USB, universal
serial bus output, SCSI, IEEE 1394 output, or any other interface
as may be appropriate for a selected application.
FIG. 12 illustrates a system 1200 that facilitates generating a
halftone image by employing a spot function based on polygonal
tessellation. The system comprises a print engine 1202 that is
coupled to a processor 1204 that executes, and a memory 1206 that
stores computer-executable instructions for performing the various
functions, methods, techniques, steps, and the like described
herein. The processor 1204 and memory 1206 may be integral to each
other or remote but operably coupled to each other. In another
embodiment, the processor 1204 and memory 1206 are integral to the
printer 1202. In another embodiment, the processor and memory
reside in a computer (e.g. the computer 150 of FIG. 1) that is
operably coupled to the printer 1202.
According to one embodiment of the subject application, the system
1200 comprises the processor 1204 that executes, and the memory
1206 that stores one or more computer-executable modules (e.g.
programs, computer-executable instructions, etc.) for performing
the various functions, methods, procedures, etc., described herein.
Additionally, "module," as used herein, denotes a set of
computer-executable instructions, software code, program, routine,
or other computer-executable means for performing the described
function, or the like, as will be understood by those of skill in
the art. Furthermore, or alternatively, one or more of the
functions described hereinafter with respect to the modules may be
manually performed.
The memory 1206 may be a computer-readable medium on which a
control program is stored, such as a disk, hard drive, or the like.
Common forms of non-transitory computer-readable media include, for
example, floppy disks, flexible disks, hard disks, magnetic tape,
or any other magnetic storage medium, CD-ROM, DVD, or any other
optical medium, RAM, ROM, PROM, EPROM, FLASH-EPROM, variants
thereof, other memory chip or cartridge, or any other tangible
medium from which the processor can read and execute. In this
context, the systems described herein may be implemented on or as
one or more general purpose computers, special purpose computer(s),
a programmed microprocessor or microcontroller and peripheral
integrated circuit elements, an ASIC or other integrated circuit, a
digital signal processor, a hardwired electronic or logic circuit
such as a discrete element circuit, a programmable logic device
such as a PLD, PLA, FPGA, Graphical card CPU (GPU), or PAL, or the
like.
The memory 1206 stores the above-identified moire-free color
conditions 1212. Upon receipt of image data 1208, which is capable
of received or generated by the processor 1204, a scanning
component (not shown) of the printer 1202, via a network connection
form a suitable image source, or the like. A halftone screen
identification module 1214 identifies a screen corresponding to one
of the colorants associated with the printer 1202 or display (not
shown) associated with the user interface 1210. The halftone
screens identified by the module 1214 executed by the processor
1204 include corresponding halftone cells in varying shapes, sizes,
and the like, as will be appreciated by those skilled in the art.
The skilled artisan will further appreciate that the halftone
screens identified may be previously calculated screens, dependent
upon the input image data, or the like.
The processor 1204 executes a frequency vector identification
module 1216 to identify the fundamental frequency vectors
associated with the halftone cell of the identified halftone
screens. An offset value selection module 1218 is then executed by
the processor 1204 so as to select a suitable offset for
application to the halftone screen for avoidance of moire
patterning. In accordance with one embodiment of the subject
application, the offset values are capable of being generated in
accordance with operations of the phase clock 1220, the raster
output scanner 1222, or the like. According to another embodiment
of the subject application, the offset value is selected in
accordance with an offset distance, such that a desired output
screen is identified and the offset value is determined based upon
the desired screen. For example, modifying the angle associated
with identified frequency vectors results may result in a suitable
offset value, similarly shifting one end of the vector along one
axis may result in a suitable offset value.
The frequency vectors identified in accordance with the operations
of the processor 1204 are then subjected to the moire-free
conditions module 1212 using the selected offset value, so as to
generate a corresponding sheared halftone screen. The processor
1204 then repeats this process for each colorant available for the
output of the image data 1208 by the printer 1202, for display via
the user interface 1210, or the like. According to one embodiment
of the subject application, the generated halftone screens are
stored in the memory 1206 for later access and subsequent use by
the processor 1204 in generating halftoned images that are
moire-free.
Once the frequency vectors have been sheared and the corresponding
halftone screen generated, the processor 1204 executes a halftone
module 1224 that uses the halftone screens as offset pursuant to
the moire-free condition module 1212, to halftone an image. Image
data 1208 is stored in the memory 1206 and may include input image
data from which an input image, intermediate image data that is
generated at various points during the described process, output
image data such as halftone image data, etc. The output image data
is provided to a print module 1226 that, when executed by the
processor 1204, generates a set of commands or instructions that
are executed by the processor 1204 and/or the printer 1202 to print
the halftone image. In another embodiment, the output halftone
image is displayed graphically on a user interface 1210 that may be
integral to the printer 1202, remote but operably coupled thereto,
or may reside on a computer such as the computer 150 of FIG. 1. In
this manner, the system 1200 can be employed to directly halftone
an image or can be used to generate a sampled version of the
halftone screen that is moire-free, which can be used to halftone
an image.
It will be appreciated that variants of the above-disclosed and
other features and functions, or alternatives thereof, may be
combined into many other different systems or applications. Various
presently unforeseen or unanticipated alternatives, modifications,
variations or improvements therein may be subsequently made by
those skilled in the art which are also intended to be encompassed
by the following claims.
* * * * *